(a) Field of the Invention
The present invention relates generally to adaptive optics systems and, more particularly, to a method for reducing the effects of scintillations on an adaptive optics system.
(b) Description of Related Art
Atmospheric turbulence, which makes the stars appear to twinkle, makes it difficult for earth-stationed optical systems to render clear images of objects outside the earth""s atmosphere. Atmospheric turbulence disrupts the wavefront of optical energy as it passes through the atmosphere. Adaptive optics systems, which are known, adjust the phase of received optical energy. When an optical signal is received, its wavefront may be uneven. That is, some segments of the wavefront may lead others. The primary function of an adaptive optics system is to align all segments of the wavefront so that the optical signal may be precisely viewed. Adaptive optics systems reflect incoming optical energy off of a deformable mirror that is divided into a number of movable zones. The adaptive optics system determines how much each segment of the wavefront is out of phase using a wavefront sensor and a reconstruction matrix. After processing the optical signals, the system generates commands, which are filtered and passed to actuators that are used to adjust the zones of the mirror into proper positions to compensate for the uneven wavefront. That is, the mirror is deformed in such a way that any wavefront segment arriving later than another actually travels a shorter distance to a focal point of the adaptive optics system. The adaptive optics system is used to alleviate the optical distortion caused by the atmosphere. This method of adaptive optics is iterated hundreds of times per second by a control system. Mirrors used in adaptive optics systems may have hundreds of movable zones. Adaptive optics systems are used in a wide variety of applications such as telescopes, laser control and guidance systems, and optical communication systems.
In addition to disrupting the phase of the optical wavefront, atmospheric turbulence causes fluctuations in the amplitude of optical signals received through the atmosphere. These amplitude fluctuations are referred to as scintillations. Scintillations may significantly degrade the performance of an optical system in comparison to the same system without scintillations. The performance measure of an optical system is the Strehl ratio, which is the ratio of received signal quality with an atmosphere to the received signal quality without an atmosphere. The Strehl ratio of an optical system may be degraded by as much as 90% when scintillations are present. Scintillations have a more severe impact on systems using coherent optical energy (e.g., lasers) than on broadband energy (e.g., energy from a star).
Historically, telescope operations have been confined to mountain-top locations to reduce the effects of scintillations. Additionally, telescope operation has been typically restricted to angles of elevation greater than 45xc2x0 to limit the amount of atmosphere through which the optical signal must propagate, thereby reducing the effects of scintillations on system performance. As communication and defense technology advances, the need to operate non-mountain-based optical systems at low angles of elevation has increased. These optical systems need to operate as effectively as possible, unhampered by scintillations.
A traditional method of mitigating the effects of scintillation includes signal processing using weighted sensor measurements using a least square method and a constant reconstruction matrix. Systems such as these merely absorb the errors created by scintillations provided scintillations are not very strong. This approach has proved acceptable in the past because, as noted, telescopes and other optical systems have limited operation to elevation angles above 45xc2x0 and the systems were receiving broadband optical energy. The least square method of weighting is computationally intensive and, therefore, requires substantial computing power to iterate the necessary calculations hundreds of times per second.
Another method for reducing the effects of scintillations employs a variable reconstruction matrix in combination with the least square weighting of measurements. This method, while effective in reducing scintillation effects, is computationally intensive. That is, not only must the measurement weighting be recalculated hundreds of times per second, but the reconstruction matrix must also be recalculated. It is worth noting that the computation of the reconstruction matrix, while known in the art, is not trivial even given the present state of computational ability. Additionally, the response of a system calculating both weighting factors and a reconstruction matrix is poor. That is, computational convergence for the reconstruction matrix components is fast in eliminating large errors and slow in eliminating small errors. This poor response is due to some of the roots of the characteristic equation, which determine the transient response of the control system, being very close to one another (very slow response) and some roots having real parts near one-half (fast, damped response).
Therefore, there exists a need for a computationally efficient method that eliminates the effects of scintillation on an adaptive optics system.
In one embodiment the present invention is an adaptive optics system for minimizing the effects of scintillations on images received by the adaptive optics system. The present invention includes a deformable mirror that is illuminated with optical energy, a plurality of actuators for moving portions of the deformable mirror, and a wavefront sensor comprising a plurality of subapertures for receiving optical energy that is reflected from the deformable mirror and for determining a slope and amplitude of the optical energy in each subaperture. The present invention further includes a slope weighting function in communication with the wavefront sensor for receiving the slope and amplitude information for each subaperture from the wavefront sensor and for processing the slope and amplitude information and a matrix multiplier in communication with the slope weighting function for receiving the processed slope and amplitude information and for generating control signals that control the actuators.
In an alternative embodiment the present invention may be embodied in a method for minimizing the effects of scintillations on images received by the adaptive optics system. The method includes the steps of illuminating a deformable mirror with optical energy, receiving optical energy from the deformable mirror in a plurality of subapertures, determining a slope and amplitude of the optical energy received by each subaperture, processing the slope and amplitude information to weight slope measurements, and moving portions of the deformable mirror using a plurality of actuators.
The invention itself, together with further objects and attendant advantages, will best be understood by reference to the following detailed description, taken in conjunction with the accompanying drawings.